The world paper industry operates in a competitive market in which both quality and cost are of vital concern. Because of the increasing cost of virgin paper fibers, papermakers are increasingly depending on recycled fiber to help satisfy their paper resource needs. This recycled paper must be of high quality however in order to effectively compete with virgin fibers.
Recycled paper must generally be deinked before being reused. As the name suggests, deinking is the art of removing various inks from printed or typed papers. These printing inks are commonly a mixture of pigment, varnish, metallic driers, additives (such as wax, flow agents and plasticizers), polymeric binders and solvents.
The composition of printing inks varies, of course, depending on its use. For example, carbon black, rosin and mineral oil are the main ingredients of news and magazine inks, while dyes, binders and volatile solvents are included in significant amounts to make flexographic inks, etc. Each type of ink presents unique problems for deinking technicians.
Generally, ink is printed onto paper as a film, the thickness of which varies according to the use. Since the ink film is a polymer formed either by drying or by polymerization of the ink, separation of the ink from the printed paper is a difficult and time-consuming process.
In the deinking technologies of the prior art, separation of ink from printed papers is generally accomplished in a pulper in aqueous solution. In such prior art processes, sodium hydroxide, sodium silicate and hydrogen peroxide are the major deinking chemicals used. This aqueous deinking process involves wetting the paper to soften both the paper and ink binders. Sodium hydroxide is used to provide an alkaline environment in which papers can be quickly wetted and swelled by alkali. Sodium silicate acts as a dispersant, pH buffer, and stabilizer of hydrogen peroxide. Hydrogen peroxide is used to improve the brightness of the deinked fibers. The combined chemical and mechanical forces act to separate the ink from the fibers. Additionally, any sizing, pigments, dyes, inorganic fillers and coatings in the system are dispersed into the solution and removed.
Methods of removing ink from wastepapers depend, at least in part, on the size of the ink particles. If the particle size is small, a wash deinking process may be used. When larger particles are present, flotation deinking methods are applied. In certain applications combinations of both processes are used.
In wash deinking processes, ink in the paper slurry is washed out by water. Disadvantages such as yellowing of the finished papers due to the presence of the residual alkali, large consumption of water, presence of both ink and chemicals in the effluent, sensitivity to ink particle size, and low quality of recycled fibers are typically associated with these processes. As a result, only about one-fourth of all deinking mills use this method.
In flotation deinking systems, collectors such as fatty acids, calcium soaps and synthetic polymers are utilized to make nonfibrous particles hydrophobic. These collectors are adsorbed to the ink particles, where they attach to air bubbles in a flotation cell and are removed by floatation from the fiber slurry. Although flotation deinking systems do not typically exhibit the problems associated with wash deinking process, such systems are limited to certain types of ink. For example, xerographic, laser printed paper, and UV cured inks can benefit from this process, but water-based, flexographic inks are preferably not deinked in this manner. Further, flotation deinking typically results in the loss of between about 10% and 15% of the paper fibers.
A need therefore exists for an improved method of deinking wastepaper which is both economical and environmentally responsible. The present invention addresses that need.